The Kureug KDuo is notorious for this.
There are many post about this.
It has to be 8nsertee perfectly in the TOP track. I fiddled for 10 minutes today. I was about to return it to store when my daughter-in-law came in and showed me.
The back patrusion on the tray has to go perfectly INTO the slide at top of coffee maker. If it's under the tray will jam up.
Itsca Definite Design Flaw I expect Keurig will eventually change.
FYI those posting "I've never had this problem" .... Then why are you posting Anything in an Answer Column? Geeeze!